With a varying demand for mobile communication traffic, higher user-plane delay performance required for future mobile communication systems has been defined by several organizations such as the International Telecommunication Union (ITU). In order to shorten a user-plane delay, data can be transmitted in a shorter slot (less than 1 ms) to shorten a period of time for processing by a base station and a user equipment (UE) and a period of time for transmission in the slot. If existing uplink scheduling and feedback tuning is still applied, it is likely that the UE is unable to make a timely response to uplink scheduling signaling, and that the base station is also unable to make feedback for or schedule re-transmission of uplink data in time.
A frame structure in a Long Term Evolution (LTE) system is as follows.
In the LTE system, a 10 ms radio frame and a 1 ms sub-frame are defined in both the Frequency Division Duplex (FDD) mode and the Time Division Duplex (TDD) mode. Seven TDD uplink-downlink configurations are defined for the radio frame in the TDD mode as depicted in Table 1 below, where D represents a downlink (DL) sub-frame, U represents an uplink (UL) sub-frame, and S represents a special sub-frame in the TDD system. Downlink data in a sub-frame are transmitted in a slot of 1 ms.
TABLE 1TDD uplink-downlink configurationsUplink-downlinkSub-frame No.configuration01234567890DSUUUDSUUU1DSUUDDSUUD2DSUDDDSUDD3DSUUUDDDDD4DSUUDDDDDD5DSUDDDDDDD6DSUUUDSUUD
Scheduling timing of a Physical Uplink Shared Channel (PUSCH) in the LTE is as follows.
In the LTE TDD system, for the TDD uplink-downlink configurations 1 to 6 and conventional Hybrid Automatic Repeat Request (HARQ) operations, the UE adjusts corresponding PUSCH transmission in a sub-frame n+k (k is given in Table 2) according to indications of Physical Downlink Control Channel (PDCCH) transmission and Physical HARQ Indicator Channel (PHICH) transmission having the Downlink Control Information (DCI) format 0 or 4, detected in the sub-frame n.
For the TDD uplink-downlink configuration 0 and the conventional HARQ operations, if the Most Significant Bit (MSB) of an uplink index in the DCI format 0 is set to 1, or a PHICH is received over a corresponding resource IPHICH0 in the sub-frame n=0 or 5, then corresponding PUSCH transmission in the sub-frame n+k (the value of k is given in Table 2) is adjusted. For the TDD uplink-downlink configuration 0, and the conventional HARQ operations, if the Least Significant Bit (LSB) of the uplink index in the DCI format 0 in the sub-frame n is set to 1, or a PHICH is received over a corresponding resource IPHICH=0 in the sub-frame n=0 or 5, or a PHICH is received in the sub-frame n=1 or 6, then the UE adjusts corresponding PUSCH transmission in the sub-frame n+7. For the TDD uplink-downlink configuration 0, if both the MSB and the LSB of the uplink index in the DCI format 0 in the sub-frame n are set to 1, then the UE adjusts corresponding PUSCH transmission in the sub-frame n+k (the value of k is given in Table 2) and the sub-frame n+7.
TABLE 2Values of k in TDD uplink schedulingUplink-downlinkSub-frame No.configuration01234567890464616464244344444454677775
In the LTE system, a plurality of radio frames are arranged in a sequence, and Table 2 illustrates the values of k corresponding to each downlink sub-frame in one radio frames by way of an example, where if n+k>9, then n+k represents the (n+k−9)-th downlink sub-frame in a succeeding radio frame.
PUSCH HARQ feedback timing in LTE is as follows.
In an LTE FDD system, the UE detects PUSCH transmission in the uplink sub-frame n−4, and feeds back PHICH information over a corresponding PHICH resource in the downlink sub-frame n.
In the LTE TDD system, for the TDD uplink-downlink configurations 1 to 6, the UE detects PUSCH transmission in the uplink sub-frame n−k′, and feeds back PHICH information over a corresponding PHICH resource in the downlink sub-frame n, where k′ is as depicted in Table 3.
In the LTE TDD system, for the TDD uplink-downlink configuration 0, the UE detects PUSCH transmission in the uplink sub-frame n−k, and feeds back corresponding PHICH information over a PHICH resource corresponding to IPHICH=0 in the downlink sub-frame n, where k′ is as depicted in Table 3; or the UE detects PUSCH transmission in the uplink sub-frame n−6, and feeds back corresponding PHICH information over a PHICH resource corresponding to IPHICH=1 in the downlink sub-frame n.
TABLE 3Values of k′ in TDD uplink feedbackTDDuplink-downlinkSub-frame no.configuration01234567890747414646266366646656664746
Round Trip Time (RTT) in the LTE system is described as follows.
The RTT is defined a period of time for a data packet to be transmitted at a time in an HARQ process and includes the following process: a transmitter starts to transmit a data packet, a receiver receives, processes the data packet and then feeds back an Acknowledgment (ACK) or a Non-Acknowledgement (NACK) signal according to a processing result, the transmitter demodulates the ACK or NACK signal and then decides to retransmit the data packet or to transmit a new data packet, in the next frame. As illustrated in FIG. 1, Tp represents a unidirectional propagation delay, Tue represents a processing delay after the UE receives feedback from an evolved Node B (eNB), and Tenb is a processing delay after the eNB receives uplink data. Tdelay1 represents a period of time for which the eNB must wait until the next downlink slot after it has processed an uplink transport block in the TDD frame structure. Tdelay2 represents a period of time for which the UE must wait until the next uplink slot after it has processed the feedback from the eNB in the TDD frame structure.
The FDD system has uniform RTT, and the RTT in the TDD system is dependent upon a particular uplink-downlink configuration.
A user-plane delay in the LTE system is introduced as follows.
As defined in Section B.2 of the appendix in 3GPP TR36.912, the user-plane delay in the LTE system includes a period of time for processing by the base station, a period of time for frame alignment, slot time, and a period of time for processing by the UE, where the period of time for frame alignment is a period of time for waiting between arrival of traffic and availability of an air-interface sub-frame for transmission of the traffic. If HARQ re-transmission in the system is taken into account, then the user-plane delay in the LTE system further includes a period of time for re-transmitting data.
In summary, when data is transmitted in a short slot in the LTE system, since a data packet transmitted at a time is small, the UE can prepare for uplink data transmission more quickly after reception of the scheduling signaling, and the base station can also demodulate the received data packet more quickly. If the existing uplink data scheduling and feedback timing is still applied, then the UE cannot make a timely response to the uplink scheduling signaling, and the period of time for feedback and re-transmission scheduling by the base station may be extended.